1. Field of the Invention
The invention is concerned with photovoltaic generation.
2. Description of the Prior Art
Photovoltaic devices have been available in a variety of commercial forms for some years. Commercially available devices have invariably depended upon a junction effect--that is, upon the junction field separation of photo-generated electron-hole pairs; and as a consequence, maximum generated voltages are, in magnitude, equal to the bandgap of the material. One such class of devices is exemplified by the selenium cell which has, for many years, been used as a photographic light meter. Voltages in the selenium cell are generated at the selenium-metal interface defined at one electrode.
A development which grew out of the emphasis on semiconductor technology stimulated by the transistor takes the form of the silicon solar cell. Here, a planar junction between p and n regions serves to divide the carrier pairs again generated within carrier lifetime distance of the junction by light absorbed in a thin outer region, and the voltage generated is equal to the approximate 1.1 volt silicon bandgap. Junction devices of this nature are described in 25, Journal of Applied Physics, page 676 (1954).
A second type of photovoltage, also limited to bandgap value may occur in the absence of a junction. The responsible effect, the "Dember effect" depends on an absorption gradient in a uniform medium. See 109, Journal of the Electrochemical Society, page 688 (1962). An analogous effect depends on a composition gradient--see 29, Reviews of Modern Physics, page 308 (1957).
Anamolously high photovoltages have been observed. The best known example concerns zinc sulfide, and a number of technical papers reporting voltages well above the bandgap have appeared. See, for example:
102, Physical Review, No. 3, page 705 et seq. May (1956);
109, Physical Review, page 1860 (1958 ),
109, Physical Review, page 601 (1958),
113, Physical Review, page 1204 (1959), and
40, Journal of Applied Physics, page 66 (January 1969). Workers have considered these anamolous effects mystifying and have attributed them to a variety of mechanisms. For example, in zinc sulfide, the effect has been attributed to junctions resulting from the stacking faults. These crystalline faults, generally lamalar in form, define interfaces between successive Wurtzite and cubic forms which coexist at room temperature.